1. Field of the Invention
This field of art to which the claimed invention pertains is solid-bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of ethanol from a feed mixture comprising ethanol and water which process employs a solid adsorbent which selectively removes the ethanol from the feed mixture.
2. Description of the Prior Art
Diminishing world supplies and availability of crude oil as well as sporadic regional shortfalls of gasoline for motor fuel have created considerable incentive for the development and use of alternative fuels. Ethanol is gaining wide popularity as such a fuel, particularly when mixed with gasoline to form a mixture known as gasohol. Gasohol may contain up to about 10 volume percent ethanol, without modifications to presently used automobile engines being required, thereby extending the volume of motor fuel availability by a like percentage.
The primary source of the ethanol used in gasohol is derived primarily from the fermentation of mash, usually from corn. Natural fermentation is able to produce an ethanol-water product mixture containing at the most about 12 vol.% ethanol. It is therefore necessary to concentrate the ethanol by distillation which, of course, requires a great amount of energy, and, in fact, the greatest cost in production of ethanol by fermentation is the energy required to separate the ethanol from the water by distillation. A means of achieving this separation without such a great expenditure of energy would thus be extremely valuable. One, therefore, might consider the many known adsorptive separation processes known in the art for possible application to the separation of ethanol from water.
For example, it is well-known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon species from mixtures thereof. The separation of normal paraffins from branched chain paraffins, for example, can be accomplished by using a type A zeolite which has pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat. Nos. 3,265,750 and 3,510,423, for example, disclose processes in which large pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Pat. No. 3,114,782, for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267 a particular zeolite is used to separate specific alkyl-substituted naphthalenes. In processes described in U.S. Pat. Nos. 3,558,732, 3,686,342 and 3,997,620, adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers. In the last mentioned processes the adsorbents used are para-xylene selective; para-xylene is selectively adsorbed and recovered as an extract component while the rest of the xylenes and ethylbenzenes are all relatively unadsorbed with respect to para-xylene and are recovered as raffinate components. Also, in the last mentioned processes the adsorption and desorption may be continuously carried out in a simulated moving bed countercurrent flow system, the operating principles and sequence of which are described in U.S. Pat. No. 2,985,589.
Unfortunately, with the adsorbents of the above processes separation of ethanol from water would be out of the question because all of those adsorbents are hydrophilic, i.e. they would be selective for water over the ethanol. Thus, in using any of these adsorbents it would be necessary to extract the water which is the major component and reject the ethanol into the raffinate. Also, there would be the problem of what could be used as an effective desorbent. The separation of the desorbent, if possible, from the ethanol raffinate and water extract would be considerably more costly than the primary distillation of the alcohol from the water.
In U.S. Pat. No. 2,474,170 mention is made that activated carbon may be employed as an adsorbent to separate the metabolism products from fermentation processes including the process producing ethyl-alcohol. This reference suggests one of two means for recovering the adsorbed components; (a) treatment of the adsorbent with steam, or (b) heating the adsorbent in vacuo. The first mentioned means would, of course, contaminate the extract stream with water, which would at least partially defeat the purpose for the extraction to begin with, and the latter means would require the costly input of heat and the very costly providing of a vacuum.
We have discovered a hydrophobic adsorbent selective for ethanol over water and a unique means for recovering the ethanol which is particularly suited for the subsequent use of the extract stream in gasohol.
In brief summary, the present invention is, in one embodiment, a process for separating ethanol from a feed mixture comprising ethanol and water. The process comprises contacting, at adsorption conditions, the mixture with an adsorbent comprising activated carbon, selectively adsorbing the ethanol to the substantial exclusion of water, and thereafter recovering high purity ethanol by treating the adsorbent with a desorbent material having utility as a motor fuel ingredient, thereby enabling the extract stream to be used directly for motor fuel blending.
In another embodiment the present invention is a process for separating ethanol from a feed mixture comprising ethanol and water which process comprises contacting at adsorption conditions the mixture with an adsorbent comprising activated carbon which process comprises the steps of: (a) maintaining net fluid flow through a column of the adsorbent in a single direction, which column contains at least three zones having separate operational functions occurring therein and being serially interconnected with the terminal zones of the column connected to provide a continuous connection of the zones; (b) maintaining an adsorption zone in the column, the zone defined by the adsorbent located between a feed inlet stream at an upstream boundary of the zone and a raffinate outlet stream at a downstream boundary of the zone; (c) maintaining a purification zone immediately upstream from the adsorption zone, the purification zone defined by the adsorbent located between an extract outlet stream at an upstream boundary of the purification zone and the feed inlet stream at a downstream boundary of the purification zone; (d) maintaining a desorption zone immediately upstream from the purification zone, the desorption zone defined by the adsorbent located between a desorbent inlet stream at an upstream boundary of the zone and the extract outlet stream at a downstream boundary of the zone; (e) passing the feed stream into the adsorption zone at adsorption conditions to effect the selective adsorption of the ethanol by the adsorbent in the adsorption zone and withdrawing a raffinate outlet stream from the adsorption zone; (f) passing a desorbent material into the desorption zone at desorption conditions to effect the displacement of the ethanol from the adsorbent in the desorption zone, the desorbent material having utility as a motor fuel ingredient; (g) withdrawing an extract stream comprising the ethanol and desorbent material from the desorption zone, the extract stream having the capability of being used directly for motor fuel blending; and (h) periodically advancing through the column of adsorbent in a downstream direction with respect to fluid flow in the adsorption zone, the feed inlet stream, raffinate outlet stream, desorbent inlet stream, and extract outlet stream to effect the shifting of zones through the adsorbent and the production of extract outlet and raffinate outlet streams.
Other objectives and embodiments of the invention encompass details about feed mixtures, adsorbents, desorbent materials and operating conditions, all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.